The present invention relates to a cluster type multi-roll rolling mill with 20 rolls and a rolling method thereby.
Special steel represented by stainless steel is on the way to expanding in an amount of production mainly for civil requirements because of its excellent corrosion resistance. Since stainless steel has the strength 2 or 3 times as strong as the strength of plain carbon steel, rolling of its thin gauge strip is difficult by an ordinary rolling mill and a rolling mill is needed in which work rolls of minimum diameter are applicable.
A typical rolling mill in which this is realized is a cluster type 20-high sendzimir rolling mill. Although there is a 12-high sendzimir rolling mill, the 20-high sendzimir rolling mill is regularly used for stainless steel of special steel.
The construction is described in detail in U.S. Pat. No. 2,776,586. The sendzimir rolling mills in which work rolls of minimum diameter are applied have increased drastically a production amount of stainless steel strip.
Further, housings of multi-roll rolling mills are roughly classified into a mono-block housing type and a split housing type. JP A 54-1259 discloses a multi-roll rolling mill in which all 4 backing bearing shafts of a lower portion are accommodated in a common frame and moved up and down.
The 20-high sendzimir rolling mill which is a cluster type multi-roll rolling mill has large advantages, caused by a mono-block housing, one intermediate roll shift, strip crown/shape control called AS-U system. However, it has the following demerits.
(1) A roll gap between upper and lower work rolls is about 6-8 mm although it depends on the diameter of the work rolls, and it was impossible to make the gap larger. PA1 (2) Restriction in strip crown/shape control ability: PA1 (3) It is impossible to measure rolling load. PA1 (1) Mill stiffness is less. PA1 (2) Sliding resistance increases and hysteresis at the time of screw-down also increases.
This is because screw-down control is effected by utilizing the eccentricity of bearings supporting backing bearings. For this reason, it is not easy to pass a rolling material through the gap. Further, in case where the rolling material is broken, much labor is spent for taking out the rolling material wound therein. Irrespective of high mill stiffness, the gap between upper and lower work rolls is restricted to be small because of geometrical dimensional relations, so that uses for its rolling are restricted.
In conventional sendzimir rolling mills, since backing bearings of an AS-U system and a screw-down device are common, a shaft of backing bearings is a common mono-block shaft to the both, and the deflectability of the shaft is limited, which results in limiting of a control amount of the AS-U.
Further, since regarding the screw-down control, only simultaneous screw-down is possible, a leveling operation has to be done by the AS-U system, so that the ability of strip crown/shape control is reduced by that extent.
Therefore, a real state of rolling operation can not be precisely taken, and a skilled art is required for precise detection of an optimum rolling condition. Further, a gauge meter system which is a useful method of automatic gauge control can not be applied.
Further, the conventional technique disclosed in JP A 54-1259 has the following disadvantages.
As shown in FIG. 4, a large load of about 55% of a rolling load is applied on each frame fillet portion 22 by component force 23 of the rolling load. However, since the rolling mill is not made of a mono-block housing, the frame fillet portions 22 are bulged and deformed by the above-mentioned large load to provide an opened upper portion, so that the mill stiffness decreases. The decrease in the mill stiffness decreases precision in thickness of a rolling strip.
Since the upper portion of the frame is opened by the above-mentioned deflection of the frame fillet portions 22, contact pressure between the frame and the housing increases, and the sliding resistance increases. As a result, hysteresis at the time of screw-down increases and preciseness in thickness of a rolling material decreases.